Research On Desigh And Fabrication Of MEMS Probe Card

Wafer level IC testing is very essential for economical manufacture because unnecessary packaging costs can be avoided by rejecting defective components at the early stage. In addition, this wafer test data provides early feedback on the overall status of the fabrication process, so that deviations can be detected and corrective action taken with minimum delay. As VLSI technology continues to evolve towards greater levels of integration and higher operating speeds, the I/O number of ICs increases and the dimension and pitch of pads shrinks, the test of these products becomes more and more important. One of the critical components that limit the test system is the probe card. The traditional needle/epoxy probe cards and the membrane probe card become unsuitable, while the MEMS probe card has no limitations of probe direction, density and precision. Furthermore, it can improve the test stability and efficiency, and minimize the influences of operator. So MEMS probe card is the developing trend of probe card technology. In this dissertation, to overcome the problems and shortcomings of current MEMS probe cards, we propose three kinds of new MEMS probe card structures and their fabrication processes, which can reduce the process steps and the fabrication costs, and possess ideal mechanical and electrical properties at the same time. The main contents and results are listed as follows:1. Study on metal cantilever type and simply supported beam type 3D MEMS probe cards fabricated by UV-LIGA technology. First, ANSYS finite elements analysis software was utilized to simulate their mechanical behavior, and the relationships between the tip displacement and Von Mises stress and the beam thickness was got. HFSS software was used to simulate the high frequency property, and the curves of S parameters of four probe structures at 1-20 GHz were achieved. Then, the cantilever type and simply supported beam type structures probe cards were fabricated by the repeated lithography and electroplating UV-LIGA process, the problems of removal of the photoresist and seed layer were solved. Last, Nano Indenter XP was used to measure the mechanical properties of the fabricated probe structures, the results show that the spring constants of cantilever type and simply supported beam type probe structures were 2556 Nm-1 and 26280 Nm-1 respectively, which were close to the designed values of 2838 Nm-1 and 23935 Nm-1 with the error of 9.94% and 8.92%. DC probes and HP 4194A Impedance/Gain-phase Analyzer were adopted to measure the electrical properties of the simply supported beam probe card. The contact resistance from the probe tip to the end of stripline was 0.6Ω. In the frequency range of 5 to 40 MHz, the characterized impedance between two probes was larger than 20 kΩ, and the capacitance was from 0.17 pF to 0.27 pF. These data proves that the designed probe cards have small contact resistance and good radio frequency isolation property.2. Study on 3D cantilever MEMS probe card structure fabricated by combining bulk silicon micromachining and UV-LIGA technology. This proposed structure can obtain the advantages of metal structure and silicon structure at the same time. First, the combining technique of bulk silicon micromachining and UV-LIGA technology was studied. 3D SU-8 photoresist standing on the silicon bump and concave microstructures as well as their metal molds and PETG replica were fabricated. Then, through theoretical analyzing the T-section, bi-materials cantilever structure, the equation that reflects the relationship between the maximum displacement and von Mises stress and the structure parameters was achieved. ANSYS finite element method was used to analyze the impact of the Si thickness, Cu thickness, Si width, Cu width and cantilever length on the maximum displacement of the composite cantilever. Last, the cantilever MEMS probe card was fabricated by the combining silicon micromachining and UV-LIGA technology, and its mechanical and electrical properties were measured by Dektak 6M surface profiler stylus, DC probes and HP 4194A Impedance/Gain-phase Analyzer. The mechanical test results agree well with the theoretical design. The probe contact resistance was only 0.035Ω, which means very low DC loss during testing. In the frequency range of 5 to 40 MHz, the characterized impedance between two probes was larger than 20 kΩ, and the capacitance was around 0.13 pF, these results show good radio frequency isolation property.3. Study on elastic substrate probe card based on PDMS and polyimide. First, COMSOL finite elements analysis software were utilized to simulate the 2D mechanical behavior of the designed structure, the relationship between the tip displacement and the PDMS and PI thickness was got. HFSS was used to simulate its high frequency property, and the curves of S parameters of four column probe structures at 1-20 GHz were achieved. Then, the PDMS elastic probe card was fabricated by MEMS fabrication technology. In the fabrication process, the single crystal silicon wet etching was used to form a step substrate with 54.7 degree angle, so as to provide the height of wire bonding and solve the problem of the interrupted line at the vertical step. Oxygen plasma treatment was adopted to improve the adhesion between PI and seed layer. Last, Nano Indenter XP was employed to measure the mechanical properties of elastic substrate and its upper probe. When the thickness of PDMS and PI was 150μm and 50μm respectively, the spring constant of elastic substrate was 4582 Nm-1, and spring constant of the upper metal probe was 7317 Nm-1. DC probes and HP 4194A Impedance/Gain-phase Analyzer were used to measure the electrical properties of the fabricated probe. The contact resistance from the probe tip to the end of stripline was 1.4Ω. In the frequency range of 5 to 40 MHz, the characterized impedance between two probes was larger than 20 kΩ, and the capacitance was from 0.19pF to 0.28 pF.